High temperature superconductivity: the state of things to come21 June 1998
HTSC technology is likely to make a significant impact in the next few years. Likely areas of application include power cables, distribution transformers, short-circuit current limiters for use with medium-voltage switchgear and energy storage systems.
The mid-1980s saw the appearance of a class of superconducting materials (initiated by the Nobel Prize winning work of Bednorz & Müller in Switzerland) that were not metallic, but oxidised ceramics. These new materials exhibit superconducting properties at much higher temperatures than "traditional" superconductor materials, in some cases as high as 100K.
These relatively high temperature superconductors do not need costly liquid helium (at a temperature of 4K) but can instead use liquid nitrogen - a cryogen that is about one-fifteenth the cost of liquid helium. This, and other factors, makes high temperature superconducting (HTSC) technology more accessible for general use and as a result a host of new energy-saving devices will see the light of day over the next ten years or so.
Transmission & distribution
The high temperature superconductors that emerged from the research laboratories of the mid-80s now provide materials with critical temperatures above the boiling point of liquid nitrogen (77K). As a result, cooling is, at least in terms of systems engineering, much more reliable, robust and cost-effective. This is because nitrogen is available in almost unlimited quantities and is relatively inexpensive to liquidise. Moreover, it is far more environmentally compatible then helium.
So far the compounds bismuth-strontium-calcium-copper-oxide (BSCCO) and yttrium-barium-copper-oxide (YCBO) are the most interesting. Consequently, the most promising are silver-tape conductors with cores of triple-bonded bismuth (2223 BPSCCO) that become superconductive at temperatures below 110K. Multicore conductors used in this way could be produced in considerable lengths using extruding machines, followed by stretching and rolling. Development work is aimed at achieving the maximum current densities over the longest possible conductor lengths. At present the peak values vary, depending on the conductor length, between 20 and 40 kA/cm2 at 77K and zero Tesla. The economic current density is 50-100 kA/cm2 over tape lengths of the order of 1km. Great efforts are being made worldwide in order to achieve this. Siemens is currently involved in three projects funded by the German Federal Ministry for Education, Science, Research and Techology (BMBF).
In the area of power cables, high temperature superconductivity can make a significant contribution in terms of minimising cost amd environmental impact. If conventional copper cables were to be replaced with HTSC lines, it would be possible, within the same space, to transmit up to five times the power yet at the same time reduce power losses. In some cases this could even eliminate 400 kV working - with a potential of 110 kV quite sufficient to transmit up to 1000 MVA. The Siemens concept, where each of the three cores contains coaxially arranged outward and return lines, ensures that no alternating magnetic fields occur in the area surrounding the cable.
Siemens recently tested 2 km of wire comprising type 2223BPSCCO tape conductors as a demonstrator cable. The current carrying capacity amounted to 3000 A and was only restricted by the power supply available at the time. The actual final value extrapolated from performance curves plotted during tests indicated capacity for 5000 A spread over four layers. This current is over twice the peak value of that carried by a conventional water-cooled copper cable when transmitting 1000 MVA. Interestingly the HTSC conductor needed only about one-twelfth of the cross-sectional area of copper compared to a conventional water-cooled 1000 MVA cable.
For the study of impedance losses and operating and fault behaviour under short term loading with AC currents up to 35 kA, the entire system was tested together with a cryostat at Siemens' Nuremberg transformer factory (one of the few locations able to provide the necessary power supply).
The AC tests were successfully concluded and a patented stranding technique enabled the world's lowest electrical losses -
0.8 W/m (at 77K) - to be achieved while carrying some 2100 A. This successful development is an important preliminary to achieving economic performance from a projected 110 kV high-capacity cable for the transmission of approximately 400 MVA, because only then will HTSC cable be able to compete with conventional copper cable despite the extra cost of cooling.
Since early 1996, Siemens subsidiary Vacuumschmelze has made valuable contributions to establishing a workable production process for HTSC technologies and this has given a great boost to Siemens' efforts to develop superconducting technology. At present, 55-filament standard wires with a current carrying capacity of 20-22 kA/cm2 can be manufactured in lengths from 400m to 600 m, and more than 30 km of such wire had been produced as of 1997. All future efforts will be directed at achieving ever higher values of maximum current density for 2223BPSCCO tapes.
Demonstrators and functional models will be important preliminary steps along the way towards the manufacture of prototype cables over the next few years. In 1998 we expect to see substantial progress made on a single-phase 50 m cable designed to operate at 110 kV. The electrical sizing of the cable cores corresponds approximately to a 400 MVA high-capacity cable.
For manufacture on a large scale it will be necessary to extend the development process to include industrial manufacturing plant and methods as quickly as possible. Siemens' Power Cables Division has already begun work on adapting existing production facilities at its Berlin factory ready for the manufacture of HTSC cable. In addition, there are many other important questions yet to be answered, such as methods for handling this type of cable, the system characteristics of HTSC cables and the advantages and disadvantages of incorporation into conventional power systems. Therefore, it should be emphasised that there is still a considerable amount of work to be done. Apart from the purely technical aspects, HTSC cable will have to prove itself conclusively against "normally conducting" and gas-insulated cables. It is anticipated that HTSC cables will be suitable for commercial use in around 2005.
In a 50 Hz alternating field at medium magnetic field strengths (about 0.3 Tesla in a transformer winding), the losses of present HTSCs are still too high. To achieve economic viability, the focus must be on further materials development. In this respect, different types of alloys are being tested for suitability as a matrix material. Techniques for the further processing of conductors need be developed for this purpose. It should be emphasised, however, that this is no trivial matter, as the superconductive fibres are made from a brittle ceramic and winding such a material presents considerable difficulties. Nonetheless, it is not an insurmountable obstacle and should be overcome within the time frames predicted.
If the system-related questions and associated technical problems arising from the use of an HTSC transformer were to be resolved here-and-now and a device built, the result would be too much of a compromise because current HTSC materials are only partially suitable. Nonetheless, a major European company has built a 630kVA HTSC distribution transformer in co-operation with American Superconductor Corp to gain further experience at an early stage of this emerging technology.
Generally, HTSC technology brings about savings in terms of space, weight and electrical losses. For stationary transformers, the savings in space and weight are only tertiary advantages, as the most important consideration is, of course, the reduction in power losses. This may seem strange at first sight, as the electrical losses of a large transformer are generally less than one per cent. However, they are nonetheless continuously present and add up to a considerable loss of energy - and, indeed, money - over time. And in these days of marginal cost considerations every little counts.
On the right track
The advantages offered are more clear cut in the case of traction transformers for high-speed trains where unnecessary weight leads to additional wear and tear and the use of more energy. With locomotion the prime criteria are compactness and low weight. Locating the drive under the floor - as is common practice with modern trains in order to save space - stretches design requirements even further. Conventional traction transformers are around 90 per cent efficient whereas an HTSC transformer of the same power rating is around 97 per cent efficient. Moreover, HTSC technology offers two further advantages: it provides increased output while occupying the same amount of space with an improved power-to-weight ratio. In future, traction transformers using HTSC technology are likely to be rated at around 10 MVA.
Siemens has, along with other companies, researched the economic practicalities of HTSC locomotive transformers. Initial results have been very encouraging and have led to further investigations.
Magnetic energy storage
A magnetic energy store has only been feasible since the advent of superconductivity. It offers a wide range of advantages, such as access time in a matter of milliseconds, high charging and discharging capacity, simultaneous provision of both active and reactive power, higher efficiency, the possibility of frequent exhaustive discharge and no moving parts. The magnetic leakage field is also well confined owing to its torus shape.
One particular field of application is the provision of seconds reserve in power transmission networks. This involves outputs ranging from 50-100 MW with an energy content of 2-3 MWh. A study sponsored by BMBF and performed by Siemens in co-operation with the utilities Preussen Elektra and RWE Energie confirmed the technical feasibility of what is commonly referred to as SMES (superconductive magnetic energy storage).
Other areas of application are protection of sensitive industrial equipment and processes against short-term voltage breakdowns and to compensate for fluctuating loads. Practical examples include semiconductor production, the manufacture of artificial fibres, rolling mills, forging operations and railways. The feasibility of this area of HTSC technology is again conditional on the progress achieved in materials development. In this respect, superconductive magnetic energy storage is competing with other methods, for example centrifugal mass storage.
Short-circuit current limiters
There are as yet no "recloseable" short-circuit limiters available for use at medium voltages, typically 10-24 kV. But such devices are possible using HTSC technology. The basic principle is simple. An ohmic or inductive resistance using an HTSC device is connected to a circuit. The device has practically no resistance as long as it is superconductive. A short-circuit current in the circuit can cause a relaxation from superconductivity to normal conductivity, thus forming a resistance. As this switching operation takes place within only 1ms, the short-circuit current is limited to between two and three times the nominal current. There will still be the need for a circuit-breaker in series with the HTSC current limiter, but it will no longer be necessary for it to be able to handle some 50 times the nominal current, as has been the case up until now. This also applies to the electrical and mechanical dimensioning of the other equipment in the circuit, eg transformers. The HTSC current limiter resets itself within about 1s, after which it is again superconductive and thus effective as a limiter.
Siemens is pursuing the concept of resistive current limiters. The inductive alternative is more readily realisable with presently available materials, but Siemens believes that it would face significant problems in larger scale operations. Yttrium-barium-copper-oxide (YCBO), which is precipitated on ceramic plates in the form of 1-3µm-thick layers, is suitable for use as the conduction material. Several plates, on which the conductor is structured in a meandering form in order to attain the required length, are connected in accordance with the required voltage and wattage rating. In a project with Canadian utility, Hydro Quebec, Siemens has successfully tested a 100 kVA device and network testing of a 10 MVA prototype is planned for 2001.
The use of superconducting current limiters is particularly desirable when other superconducting equipment such as transformers, cables or energy storage is used. With its typically short response time, a limiter protects the equipment while also preventing quenching in the equipment itself which, owing to the long cooling times involved, could lead to undesirable outages. At about 1 second, the cooling time of the superconducting current limiter is markedly less than that of the other equipment. A superconducting current limiter brings benefits whensystems are growing rapidly they are deeply meshed new power stations are to be added to the network and there are high concentrations of load, e.g. in cities and industrial power supplies.
Opportunities and questions
It has been estimated that worldwide sales of superconductors by the early part of the next century could approach $30bn per annum (based on present cost estimates), accounting for about one-third of the world transmission & distribution market.
However, the application of HTSC materials needs careful consideration. The high temperature superconducting ceramics discovered to date contain copper, but other constituents can be very varied and they all have a complex crystalline structure. Typically, they comprise planes of atoms along which electrons flow. Consequently, their electrical conductivity has different values when measured in different directions within the crystal. Furthermore, conductivity can be critically dependent on the orientation of such planes within fabricated materials. An additional consideration is that they are brittle ceramics.
As well as posing application problems, HTSC superconductors also present theoretical questions. The most obvious being: What is the mechanism governing superconductivity? Can the Bardeen, Cooper & Schrieffer theory be adapted to explain the phenomenon, or must a new mechanism be invoked? A successful explanation could point the way to even higher temperature superconductors and perhaps, one day, materials that are superconducting at room temperature.